[BUGFIX] `ember init` fails on `NULL_PROJECT`

Previously, if you try to run `ember init` inside an empty directory, it
will fail because `environment.project` was not set to `NULL_PROJECT`.

Steps:

```bash
mkdir foo && cd foo && ember init
```

thanks to @abuiles for initial spike, [#529](https://github.com/stefanpenner/ember-cli/pull/529)

Closes #540